In circuit arrangements which have a load transistor for switching or regulating a current flowing through a load, it may be necessary to measure a load current flowing through the load transistor. The current information thereby obtained may, for example, be used to regulate the current through the load or to switch off the load transistor to protect the latter against overload if the load current exceeds a predefined threshold value.
The “current-sense principle” can be applied to measure the load current through a load transistor. Here, a measuring transistor is provided which is connected in such a way that it is operated at least approximately at the same operating point, i.e., at the same control voltage, as the load transistor. A current flowing through the measuring transistor is then directly related to the load current and is evaluated to determine the load current. The ratio between the measuring current and the load current corresponds to the ratio between the total channel cross-section of the measuring transistor and the total channel cross-section of the load transistor or the ratio between the active transistor area of the measuring transistor and the active transistor area of the load transistor.
If the load transistor and the measuring transistor are not operated at exactly the same operating point, because, for example, an offset exists between the control voltage of the load transistor and the control voltage of the measuring transistor, the measuring current is not exactly proportional to the load current. However, a resulting measurement error is negligible if the flowing load current is so high that the control voltage of the load transistor is very much higher than the offset. Conversely, in the case of low load currents, there is a risk that the offset will lie in the region of the control voltage, so that a measurement error is no longer negligible.